When operating a pitch controlled wind turbine, the pitch angle of the blades is controlled in order to maximise the rotor power coefficient, CP, without exceeding the nominal power coefficient, CP, nom. Thus, at relatively low wind speeds, the so-called ‘partial load’ region, the pitch angle is controlled in such a manner that the power coefficient is as high as possible, and at higher wind speeds, the so-called ‘full load’ region, the pitch angle is controlled in such a manner that the power coefficient is limited to the nominal power coefficient, CP, nom. To this end theoretical curves are used for calculating the pitch angle to be applied under the given circumstances. Such curves could, e.g., specify the optimal pitch angle as a function of tip speed ratio, λ, the optimal pitch angle in this context being the pitch angle which provides the highest possible power coefficient, CP. The tip speed ratio, λ, is the ratio between the blade tip speed and the wind speed.
In some wind regimes, in particular at wind speeds just below the wind speed where the nominal power coefficient, CP, nom, is reached, there is a risk that the rotor blades stall and the airflow over the blades separates from the blade, if the turbine is operated in accordance with the ‘optimal pitch’ curves described above, e.g. in the case of turbulence or strong wind shear. Since this is very undesirable, a more precautious control of the pitch angle must be selected, at least in this wind region. Furthermore, the precautious control of the pitch angle is performed in a manner which prevents stalling of the blades as well as excessive loads on various components of the wind turbine, even in a ‘worst case scenario’. As a consequence, the wind turbine is operated in a sub-optimal manner, and the power production of the wind turbine is reduced as compared to a theoretical maximum power production.
In Zhang, X., et al., “Intelligent control for large-scale variable speed variable pitch wind turbines”, Journal of Control Theory and Applications 3 (2004) 305-311, a method for operating a wind turbine is disclosed. The proposed control system consists of two loops: An inner loop for generator torque regulation using adaptive fuzzy control based on variable universe to maximise the capture of power at below-rated wind speed; an outer rotor loop to control pitch angle and keep rated power using least square support vector machine (LS-SVM) control. The rotor loop controller input variables are the generator output power, the shaft rotational speed, ω, and the estimated effective wind speed, Vw. The output control variable is the reference pitch angle, βref, of the rotor blades.